Cytochrome P-450 (P-450) monooxygenases are ubiquitous hemoproteins present in microorganisms, plants and animals. Comprised of a large and diverse group of isozymes, P450s mediate a great array of oxidative reactions using a wide range of compounds as substrates, and including biosynthetic processes such as phenylpropanoid, fatty acid, and terpenoid biosynthesis; metabolism of natural products; and detoxification of foreign substances (xenobiotics). See e.g., Schuler, Crit. Rev. Plant Sci. 15:235-284 (1996). In a typical P-450 catalyzed reaction, one atom of molecular oxygen (O.sub.2) is incorporated into the substrate, and the other atom is reduced to water by NADPH. For most eucaryotic P-450s, NADPH:cytochrome P-450 reductase, a membrane-bound flavoprotein, transfers the necessary two electrons from NADPH to the P-450 (Bolwell et al, Phytochemistry 37: 1491-1506 (1994)).
Frear et al. (Phytochemistry 8:2157-2169 (1969)) demonstrated the metabolism of monuron by a mixed-function oxidase located in a microsomal fraction of cotton seedlings. Further evidence has accumulated supporting the involvement of P-450s in the metabolism and detoxification of numerous herbicides representing several distinct classes of compounds (reviewed in Bolwell et al., 1994; Schuler, 1996). Differential herbicide metabolizing P-450 activities are believed to represent one of the mechanisms that enables certain crop species to be more tolerant of a particular herbicide than other crop or weedy species.